The useful life of a perishable product is a function of its cumulative thermal exposure, which is a combination of the temperatures to which the product is exposed and the duration of the exposure. Degradation reactions occur faster at higher temperatures than they do at lower temperatures. Therefore, a perishable product will have a longer useful life if it is exposed to lower temperatures than if it is exposed to higher temperatures. Perishable products include, but are not limited to, food, food additives, chemicals, biological materials, drugs, cosmetics, photographic supplies and vaccines.
Many manufacturers mark their products with printed expiration dates in an attempt to provide an indication of when the useful life of a perishable product ends. However, these dates are only estimates and are unreliable because they are based on assumptions about the thermal history of the product that may not be true with respect to a particular package on which they appear. Namely, in computing expiration dates, a manufacturer assumes that during its useful life a product will be kept at temperatures within a specific range prescribed for best results. However, if the actual temperatures of exposure are higher than those used in calculating the printed expiration date, the perishable item may degrade or spoil before the marked expiration date. In such a case, the printed expiration date would mislead a consumer into believing the product was still usable when in fact it was past its useful life.
A time-temperature integrating indicator that gives a visually observable indication of the cumulative thermal exposure of a specific item, and hence overcomes the problems inherent with the use of marked expiration dates, is disclosed in U.S. Pat. No. 5,667,303, entitled "Time-Temperature Integrating Device," issued to Arens et al. (the '303 patent). The device of the '303 patent includes a first laminate wherein a substrate is coated with an opaque, porous matrix, and a second laminate having a backing material coated with a viscoelastic indicator material. The viscoelastic material and the porous matrix have the same, or approximately the same, indexes of refraction. The device is activated by placing the viscoelastic material and the substrate in contact with each other and mounting the combination on an object whose cumulative thermal exposure is to be monitored. The viscoelastic material progressively migrates into the porous matrix at a rate that increases with increasing temperature. As the microvoids of the opaque, porous matrix become filled with viscoelastic material, the porous matrix becomes transparent. The device thereby provides a visually observable indication that a predetermined cumulative thermal exposure associated with a change in the object (such as degradation or spoilage) has been met.
The visually observable indication in the device of the '303 patent occurs when the cumulative thermal exposure of the device is equal to the cumulative thermal exposure required to cause the degradation or other change being monitored in the test object. The viscoelastic material used in the indicator of the '303 patent is selected so that the run out time of the indicator, which is the time needed for the indicator to provide a visually observable indication, matches the time needed for the monitored change in the object to occur.
Matching the temperature dependence of the rates of change in the indicator device and the monitored object is accomplished by matching the Q10 or Ea of the viscoelastic material with the Q10 or Ea of the monitored object, according to the method described in the '303 patent. Q10 and Ea are both related to the temperature dependence of the rate of change of an object. Q10 is an indication of how much faster a reaction occurs in response to a 10.degree. C. increase in temperature. Ea, or activation energy, is computed with reference to the Arrhenius Equation, K=Ko exp (-Ea/RT), where K=the rate constant at temperature T, Ko=the preexponential factor, R=the ideal gas constant and Ea=activation energy.
The first laminate and second laminate used in the indicator devices of the '303 patent are stored in separate rolls until needed for use. The indicator devices are constructed by cutting a length from each roll and placing the viscoelastic material of the second laminate in contact with the porous matrix of the first laminate.
One shortcoming of the indicator devices of the '303 patent is that some viscoelastic materials, in addition to migrating into the porous matrix, tend to flow laterally between the layers of the storage rolls and the indicator devices. In the storage rolls, the viscoelastic material flows laterally between the layers in the roll and forms a gooey accumulation on the sides of the rolls. This accumulation interferes with the operation of the laminating machine used to make the indicators Furthermore, in the indicators the viscoelastic material flows out between the substrate and the backing and creates a sticky mess on both the indicator and the object to be monitored. As a result of the lateral flow of the viscoelastic material, the thickness of the viscoelastic material on the backing is rendered nonuniform.
Therefore, what is needed is a means to retain the advantages of the time temperature integrating indicators described in the '303 patent while preventing the viscoelastic material from flowing laterally between the backing and the substrate and oozing out of the indicator.